There are many industrial applications that require a thin ceramic layer or layers be formed on a substrate. For instance, inorganic membranes possess thin porous ceramic layers supported by porous substrates. Oxygen transport membranes are another class of applications in which a thin dense layer of an oxygen ion conducting material is supported on one or more porous substrates. The substrates can be metal or a ceramic. Generally, the ceramic material used in an oxygen transport membrane is a perovskite that is capable of conducting both oxygen ions and electrons at elevated temperatures and upon application of a partial pressure differential to separate oxygen from an oxygen containing feed. Certain ceramic materials are capable of conducting protons and can similarly be used for hydrogen separation. A closely related art involves fuel cells that utilize a very thin electrolyte layer that is formed of a ceramic that is also capable of conducting oxygen ions. The electrolyte layer is sandwiched between electrodes to conduct electrons generated through ionization of the oxygen and recombination of oxygen ions to a load.
Several methods have been developed forming ceramic layers on substrates, including organometallic chemical vapor deposition, electrochemical vapor deposition, colloidal/slurry coating, tape isopressing, sol-gel and plasma spray. U.S. Pat. No. 5,439,706 discloses an organometallic chemical vapor deposition method to prepare thin films on multicomponent and oxides for use in inorganic membranes. Such a process involves expensive and complex equipment and often the use of toxic and expensive precursor materials.
Plasma spraying is a fast, cost-effective method for fabricating dense oxygen or hydrogen transport membrane coatings that are free of microcracks on dense or porous substrates. Plasma spraying involves spraying a molten powder of metal or metal oxide onto the surface of a substrate using a plasma spray gun. An example is shown in U.S. Pat. No. 6,638,575. The problem with plasma spraying is that it is not easy to achieve very thin gas tight dense layers on porous supports.
U.S. Pat. No. 6,524,421 discloses a tape isopressing method that combines conventional tape casting and cold isopressing techniques to fabricate oxygen transport membranes that utilize a porous substrate to support a dense layer. A tape containing the ceramic is applied to the substrate and a green form is produced by the cold isopressing. The green form is then fired to produce the finished article. Colloidal/slurry coating techniques involve the formation of a slip containing the ceramic particles to be applied. In such method, a porous substrate to serve as a support is dipped in the slip to form a thin coating that can subsequently be processed from its green state into a thin dense layer. The disadvantage of both of such techniques is that they require a close matching of shrinkage between the coating and the substrate upon firing. In the absence of such close matching the dense layer will have cracks and be incapable of performing in its intended function.
As will be discussed the present invention provides a method that allows very thin, film-like layers to be formed on substrate without expensive equipment or the use of potentially toxic precursors with the result that that the ceramic layer or layers incorporated into a composite structure are formed in the finished article without cracks or other major defects.